1. Field of the Invention
The present invention relates generally to a caster and more specifically to a hubless caster for use in office furniture (chairs, tables, desks) as well as other devices or conveyances used primarily for transport of materials and pushed by hand (grocery carts, wagons). This new device is a caster that allows better inherent stability, easier rolling, and affords easy customization to enhance the overall aesthetic of the supported object (chair, table, cart, file cabinet, or other pieces of furniture). For the purpose of this patent, the attached device will assumed to be an office chair.
2. Description of Related Art
It can be appreciated that casters have been in use for decades. Casters are located between the chair and the floor on which the chair is rolling, and are usually used in groups of 4 or 5 per end device. Typically, caster designs tend to fall into one of two groups: the ‘single wheel’ and the ‘twin wheel’. The single wheel caster consists of one wheel with an axle through the center which is attached to a support base. The support base is fixed to the chair (or other object) by use of a vertical pin that allows the caster to pivot about a generally vertical axis while also permitting rotation of the wheel about a generally horizontal axis, thereby allowing the caster to move in any direction.
The other main type of caster design is the ‘twin wheel’ caster, which is identical to the first mentioned except two wheels are used instead of one. The two wheels share a common axle, but are free to rotate separately. The axle is affixed to the vertically extending base at a location in the middle of the two wheels. In this way, each wheel is free to rotate in opposite directions, facilitating a change in direction of the chair direction. Also, the wheels may turn in the same direction for straight-line movement of the chair.
The main problem of the ‘single wheel’ caster is its inability to turn easily about the pin pivot. The wheel needs to rotate about the straight line formed by the wheel's contact with the horizontal floor. Effectively, a portion of the contacting wheel needs to drag on the floor instead of freely rolling. Additionally, the central axle running through the centerline of the wheel is supported at the outsides of the wheel. These outside locations are unprotected from any collisions with other chairs or walls, often leading to a bent axle, which seriously impairs the usability of the caster. This design is still available though is less commonly used in modern office furniture.
The ‘twin wheel’ caster offered an improvement over the ‘single wheel’ in two important regards. The ability of the wheels to rotate in opposite directions at the same time greatly enhanced the ability to turn about the vertical pivot, making a change in overall direction of the chair very smooth. Also, the support of the central axle to the chair in the middle of the two wheels afforded greater protection of the thin axles, greatly reducing the possibility of damage to the axle and subsequent loss of performance. One problem germane to this caster type is the method of attachment of the wheel to the axle. Unlike the single wheel with its axle attachment on the two faces of the wheel, the double wheel design attaches each wheel to its respective end of the axle, the axle support to the base being disposed between the two wheels. This attachment configuration creates some inherent stability inefficiencies. By not allowing the axle to go through the wheel, the loading of the wheel on the axle is not symmetric. That is, the inside of the wheel is fully loading the axle while the outside is not loading the axle at all. Also, the limited space remaining in the device for the wheel thickness results in thin wheels, which directly result in narrow annular (limited) bearing surfaces of the wheel on the axle.
The overall attachment of the wheel to the axle is inherently not completely stable. With the simple method employing a pin (axle) in a hole (in the wheel), the resulting configuration is often fairly loose and sloppy. This can be easily verified by inspection of any casters of this type. Simply moving the wheels by hand shows the amount of ‘play’ in the assembly and lack of inherent design stability. Note these effects multiply over time as the friction in the joints further moves the features from the design ideals.
Both types of casters, additionally, suffer common drawbacks. In its most common embodiment, the wheels turn on the axle, relying on sliding friction to afford rotation. This type of friction is not as smooth and efficient as other types of motion. Additionally, over time the friction removes material in the hole, creating a larger hole and subsequent ‘wobble’ as the tight fit in the axle is lost.
Also, both casters rely on the wheels transferring load to axles located at the exact centerline of the wheel(s). This, at first glance, appears to be the most rational design dating back to the original wheel-about-axle. Certainly, this configuration is most stable for wheels turning very quickly, for example bikes or motorcars. But for wheels whose primary purpose is to carry load and turn at very slow rotational speeds (5 rpm and often, for long periods of time, zero), the center axle is not ideal. Under many loading instances, the wheels impart a moment about the wheel centerline perpendicular to the direction of travel, such as in turning of the caster. This moment occurs because the force on the wheel (at the floor contact) is multiplied by the distance to the axle, the wheel radius. This moment adds more loading of the aforementioned wheel/axle joint, further decreasing the inherent stability of the overall device.
Finally, there is a common failing in casters of either of the common designs. Needless to say, a caster is of no use by itself. The sole purpose of a caster is to provide greater functionality (ease of movement) to the overall object (office chair). As a piece of a greater whole, the caster should have the ability to enhance the overall design characteristics of the chair. This may be accomplished by replicating design features from the rest of the chair, using consistent materials, or in other ways complimenting the overall intent of the chair design. With the ‘twin caster’ design, the prominent wheels with their solid center walls, located on the outside of the device dominate the appearance of the caster assembly. This leads to few opportunities to customize this type of caster for a specific chair design.
With few opportunities for differentiation in the wheel, this caster design leads to the device being featured on almost all furniture products today without sharing any design traits (materials, color, design features) with the entire chair. Caster designs remain consistently the same from chair to chair, manufacturer to manufacturer, year to year.
The relevant prior art includes U.S. Pat. No. 4,045,096, a rotor is mounted within a shroud, and various embodiments of roller bearings are shown for mounting the rotor to the shroud. In U.S. Pat. No. 4,465,321, a hubless wheel is mounted within a fender that describes less than a full circle. The wheel is collapsible, and is supported by the surrounding fender. U.S. Pat. No. 5,248,019 shows motorcycle and bicycle constructions that use hubless wheels. The wheel is apparently mounted to the fixed rim with one bearing. U.S. Pat. Nos. 5,419,619 and 5,490,719 relate to a bicycle construction. Both are directed to the precise angular spacing of the bearings mounted on the fixed hoop of a hubless wheel assembly, and appear to rely on four bearings unequally spaced bearings.